Pharmacologic characterization of the oxytocin receptor in human uterine smooth muscle cells

1 [H-3]-oxytocin was used to characterize the oxytocin receptor found in hu man uterine smooth muscle cells (USMC). Specific binding of [H-3]-oxytocin to USMC plasma membranes was dependent upon time, temperature and membrane protein concentration. 2 Scatchard plot analysis of equilibrium binding data revealed the existenc e of a single class of high-affinity binding sites with an apparent equilib rium dissociation constant (K-d) of 0.76 nM and a maximum receptor density (B-max) of 153 fmol mg(-1) protein. The Hill coefficient (n(H)) did not dif fer significantly from unity, suggesting binding to homogenous, non-interac ting receptor populations. 3 Competitive inhibition of [H-3]-oxytocin binding showed that oxytocin and vasopressin (AVP) receptor agonists and antagonists displaced [H-3]-oxytoc in in a concentration-dependent manner. The order of potencies for peptide agonists and antagonists was: oxytocin > [Asu(1,6)]-oxytocin > AVP = atosib an > d(CH2)(5)Tyr(Me)AVP > [Thr(4),Gly(7)]-oxytocin > dDAVP, and for nonpep tide antagonists was: L-371257 > YM087 > SR 49059 > OPC-21268 > SR 121463A > OPC-31260. 4 Oxytocin significantly induced concentration-dependent increase in intrac ellular Ca2+ concentration ([Ca2+](i)) and hyperplasia in USMC. The oxytoci n receptor antagonists, atosiban and L-371257, potently and concentration-d ependently inhibited oxytocin-induced [Ca2+](i) increase and hyperplasia. I n contrast, the V-1A receptor selective antagonist, SR 49059, and the V-2 r eceptor selective antagonist, SR 121463A, did not potently inhibit oxytocin -induced [Ca2+](i) increase and hyperplasia. The potency order of antagonis ts in inhibiting oxytocin-induced [Ca2+](i) increase and hyperplasia was si milar to that observed in radioligand binding assays. 5 In conclusion, these data provide evidence that the high-affinity [H-3]-o xytocin binding site found in human USMC is a functional oxytocin receptor coupled to [Ca2+](i) increase and cell growth. Thus human USMC may prove to be a valuable tool in further investigation of the physiologic and pathoph ysiologic roles of oxytocin in the uterus.